Allylation is one of the extremely important C—C bond forming reactions. Accordingly several “allyl” metal species have been extensively employed in the reaction with functional groups such as carbonyl (C═O) and imine (C═N), leading to the formation of the corresponding homoallylic alcohols or amines respectively. Among the several metals employed for allylation, allylboranes are highly unique and proceed with high stereoselectivity. The diastereoselectivity arises via a rigid six-membered chair like transition state due to the similarities in size of boron and carbon.
There have been significant advances made in the development of enantioselective versions of allylboration utilizing chiral allylboranes derived from α-pinene, tartarate, camphor etc. Of the several chiral auxiliaries employed, α-pinene is of special importance as the corresponding allylboranes result in very high enantiomeric excesses (ee) for a wide variety of aldehydes and imines (Scheme 1). See Brown, H. C.; Jadhav, P. K. J. Am. Chem. Soc. 1983, 105, 2092; Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108, 293; Brown, H. C.; Bhat, K. S. J. Am. Chem. Soc. 1986, 108, 5919; Brown, H. C.; Jadhav, P. K.; Bhat, K. S. J. Am. Chem. Soc. 1988, 110, 1535; Ramachandran, P. V., et al., Org. Biomol. Chem. 2005, 3, 3812; Ramachandran, P. V., et al., J. Org. Chem. 2004, 69, 6294; and Ramachandran, P. V., et al., J. Org. Chem. 2002, 67, 7547.

The reaction with α-pinene derived boranes is reagent controlled, and the ee of the product obtained depends on the antipode of α-pinene used, regardless of the chirality in the substrate. One such α-pinene reagent is B-allyldiisopinocampheylborane, which provides exceptional enantioselectivity for aldehydes and imines at low temperatures. Unfortunately, B-allyldiisopinocampheylborane is not commercially available, due to its perceived instability. Thus, it is typically generated in situ or freshly prepared before use as illustrated in Scheme 2.

The preparation is tedious and involves the reaction of B-methoxydiisopinocampheylborane with allylmagnesium bromide in ether at 0° C. using Brown's procedure (11B NMR shows a peak at ˜78 PPM). The reaction generates a large amount of solid methoxymagnesium bromide that needs to be filtered under inert atmosphere, followed by repeated washings with pentane to precipitate the excess Grignard reagent. The reagent thus obtained is typically used for allylboration reactions as a 1-2M solution in pentane at low temperatures (−78° C. to −100° C.).
In view of the versatility of this reagent and its enormous potential for application in pharmaceutical industry, there is a need for a stabilized form of B-allyldiisopinocampheylborane as well as other borane reagents that can be stored and sold commercially. There is also a need for improved methods for using such reagents that require less stringent handling requirements.